Fairfax County, VirginiaOwner: Fairfax County Department of Public Works and Environmental Services

Fairfax County hired Wetland Studies and Solutions, Inc. (WSSI) for this project as a result of our successful work at the Judicial Center Parking Garage rain garden (another Fairfax County project). WSSI determined the cause of failure for a 4,000 s.f. rain garden (designed and constructed by others) in front of Fire Station No. 40. Noted problems with the facility were twofold: the rain water overtopped the berm, which separated the rain garden from the adjacent Legato Road, in large rain events, and the rain garden held water for weeks, even in dry weather. These problems produced traffic and mosquito hazards, and the facility did not serve its intended function to slow and filter runoff from the fire station’s impervious surfaces. WSSI’s on-site and laboratory investigation into the problem included physical examination, surveying, soil content analysis, and falling-head infiltration tests

Utilizing existing horizontal and vertical control, WSSI surveyors performed and prepared a detailed (1” = 10’, 0.5’ contour interval) topographic survey of the site. The survey included existing curb and gutter associated with the adjacent road and parking lots. Curb inlets, yard inlets, grates, manholes, roof drains and the connecting pipes flowing into the rain garden or nearby were located. Each structure and the pipes in and out were dimensioned, and invert elevations were recorded. A complex system of reinforced concrete and PVC pipes were investigated, and connections were depicted in the finished survey drawing.

WSSI’s investigation concluded that the rain garden’s underdrain system was adequate and functioning properly and, therefore, was not the cause of the ponding problem. However, laboratory tests showed that the existing planting media had an infiltration rate of less than 0.25 inches per hour; thus, the 6-inch depth of the full rain garden would take over 24 hours to completely drain. (In contrast, the Fairfax County Public Facilities Manual specifies a minimum infiltration rate of 1.5 inches per hour.) Analysis of the planting media composition showed that the organic matter percentage was too high and was acting to block the flow of water to the underdrain system below. Another problem was highlighted by the survey of the berm which revealed that it had been built too low, causing water during large storm events to bypass the emergency overflow and spill out onto Legato Road, rather than into an existing stormwater basin.

After determining the cause of failure, WSSI prepared a rehabilitation plan, which included removal of the existing soil media; mixing the existing media with construction sand to enhance infiltration; restructuring the berm; and reconfiguring the rain garden’s inlet. WSSI also performed construction oversight during the construction process.